Acute viral hepatitis is clinically diagnosed by a well-defined set of patient symptoms, including jaundice, hepatic tenderness, and an increase in the serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase. Additional serologic immunoassays are generally performed to diagnose the specific type of viral causative agent. Historically, patients presenting clinical hepatitis symptoms and not otherwise infected by hepatitis A, hepatitis B, Epstein-Barr or cytomegalovirus were clinically diagnosed as having non-A non-B hepatitis (NANBH) by default. The disease may result in chronic liver damage.
Each of the well-known, immunologically characterized hepatitis-inducing viruses, hepatitis A virus (HAV), hepatitis B virus (HBV), and hepatitis D virus (HDV) belongs to a separate family of viruses and has a distinctive viral organization, protein structure, and mode of replication.
Attempts to identify the NANBH virus by virtue of genomic similarity to one of the known hepatitis viruses have failed, suggesting that NANBH has a distinct organization and structure. [Fowler, et al., J. Med. Virol., 12:205-213 (1983) and Weiner, et al., J. Med. Virol., 21:239-247 (1987)].
Progress in developing assays to detect antibodies specific for NANBH has been particularly hampered by difficulties in correctly identifying antigens associated with NANBH. See, for example, Wands, J., et al., U.S. Pat. No. 4,870,076, Wands, et al., Proc. Nat'l. Acad. Sci., 83:6608-6612 (1986), Ohori, et al., J. Med. Virol., 12:161-178 (1983), Bradley, et al., Proc. Nat'l. Acad. Sci., 84:6277-6281, (1987), Akatsuka, T., et al., J. Med. Virol, 20:43-56 (1986), Seto, B., et al., U.S. patent application Ser. No. 07/234,641 (available from U.S. Department of Commerce National Technical Information Service, Springfield, Va., No. 89138168), Takahashi, K., et al., European Patent Application No. 0 293 274, published Nov. 30, 1988, and Seelig, R., et al., in PCT Application PCT/EP88/00123.
Recently, another hepatitis-inducing virus has been unequivocally identified as hepatitis C virus (HCV) by Houghton, M., et al., European Patent Application publication number 0 318 216, May 31, 1989. Related papers describing this virus include Kuo, G., et al., Science, 244:359-361 (1989) and Choo, Q., et. al, Science, 244:362-364 (1989). Houghton, M., et al. reported isolating cDNA sequences from HCV which encode antigens which react immunologically with antibodies present in patients infected with NANBH, thus establishing that HCV is one of the viral agents causing NANBH. The cDNA sequences associated with HCV were isolated from a cDNA library prepared from the RNA obtained from pooled serum from a chimpanzee with chronic HCV infection. The cDNA library contained cDNA sequences of approximate mean size of about 200 base pairs. The cDNA library was screened for encoded epitopes expressed in clones that could bind to antibodies in sera from patients who had previously experienced NANBH.
In the European Patent Application, Houghton, M., et al. also described the preparation of several superoxide dismutase fusion polypeptides (SOD) and the use of these SOD fusion polypeptides to develop an HCV screening assay. The most complex SOD fusion polypeptide described in the European Patent Application, designated c100-3, was described as containing 154 amino acids of human SOD at the aminoterminus, 5 amino acid residues derived from the expression of a synthetic DNA adapter containing a restriction site, EcoRI, 363 amino acids derived from the expression of a cloned HCV cDNA fragment, and 5 carboxyl terminal amino acids derived from an MS2 cloning vector nucleotide sequence. The DNA sequence encoding this polypeptide was transformed into yeast cells using a plasmid. The transformed cells were cultured and expressed a 54,000 molecular weight polypeptide which was purified to about 80% purity by differential extraction.
Other SOD fusion polypeptides designated SOD-NANB.sub.5-1-1 and SOD-NANB.sub.81 were expressed in recombinant bacteria. The E.coli fusion polypeptides were purified by differential extraction and by chromatography using anion and cation exchange columns. The purification procedures were able to produce SOD-NANB.sub.5-1-1 as about 80% pure and SOD-NAN38, as about 50% pure.
The recombinant SOD fusion polypeptides described by Houghton, M., et al. were coated on microtiter wells or polystyrene beads and used to assay serum samples. Briefly, coated microtiter wells were incubated with a sample in a diluent. After incubation, the microtiter wells were washed and then developed using either a radioactively labelled sheep anti-human antibody or a mouse antihuman IgG-HRP (horseradish peroxidase) conjugate. These assays were used to detect both post acute phase and chronic phase HCV infection.
Due to the preparative methods, assay specificity required adding yeast or E. coli extracts to the samples in order to prevent undesired immunological reactions with any yeast or E.coli antibodies present in samples.
Ortho Diagnostic Systems Inc. have developed a immunoenzyme assay to detect antibodies to HCV antigens. The Ortho assay procedure is a three-stage test for serum/plasma carried out in a microwell coated with the recombinant yeast/hepatitis C virus SOD fusion polypeptide c100-3.
In the first stage, a test specimen is diluted directly in the test well and incubated for a specified length of time. If antibodies to HCV antigens are present in the specimen, antigen-antibody complexes will be formed on the microwell surface. If no antibodies are present, complexes will not be formed and the unbound serum or plasma proteins will be removed in a washing step.
In the second stage, anti-human IgG murine monoclonal antibody horseradish peroxidase conjugate is added to the microwell. The conjugate binds specifically to the antibody portion of the antigen-antibody complexes. If antigen-antibody complexes are not present, the unbound conjugate will also be removed by a washing step.
In the third stage, an enzyme detection system composed of o-phenylenediamine 2HCl (OPD) and hydrogen peroxide is added to the test well. If bound conjugate is present, the OPD will be oxidized, resulting in a colored end product. After formation of the colored end product, dilute sulfuric acid is added to the microwell to stop the color-forming detection reaction.
The intensity of the colored end product is measured with a microwell reader. The assay may be used to screen patient serum and plasma It is established that HCV may be transmitted by contaminated blood and blood products. In transfused patients, as many as 10% will suffer from post-transfusion hepatitis. Of these, approximately 90% are the result of infections diagnosed as HCV. The prevention of transmission of HCV by blood and blood products requires reliable, sensitive and specific diagnosis and prognostic tools to identify HCV carriers as well as contaminated blood and blood products. Thus, there exists a need for an HCV assay which uses reliable and efficient reagents and methods to accurately detect the presence of HCV antibodies in samples.